Raman Investigation of the Ferroelectric Transition in Tris-Sarcosine Calcium Chloride.

The behavior associated with structural phase transitions in crystalline solids is characterized by two extremes. At one end we find order-disorder transitions. In that case, the crystal exhibits random disorder in the high-temperature phase, and assumes increasing order below the transition. At the other end lie transitions which are described as displacive; these systems are perfectly ordered in each phase and are characterized by a shifting of atomic positions. Our discovery through Raman scattering of a soft vibrational mode in tris-sarcosine calcium chloride (TSCC), which undergoes a ferroelectric transition at approximately 127K, establishes the largely displacive character of that transformation; for nearly twenty years the ferroelectricity was believed to be primarily the result of an order-disorder mechanism. Additional Raman studies of brominated TSCC suggest the importance of a mode coupling in driving the transition. A simple model is developed to demonstrate formally how such a mechanism might yield the observed behavior. In addition, a new mean-field form for the temperature dependence of the soft mode is discussed, as well as Raman results at elevated temperatures.